Exhaust emissions from motorized on-highway vehicles are currently regulated by the Federal Government and must not exceed certain contaminant levels as is set forth in Title 40, Chapter I of The Code of Federal Regulations, Section 86 Subpart C. Because of these regulations increasingly more sophisticated testing equipment has been developed to test and analyze engines for conformance with such standards. For example, one recent regulation set by the Environmental Protection Agency (EPA) involves a particulate limit standard of 0.60 g/hph for diesel truck engines. These particulates are defined as matter in the exhaust gas stream, other than condensed water, which can be collected on a special filter after dilution with ambient air to a maximum temperature of 52.degree. C.(125.degree. F.). This includes agglomerated carbon particles, absorbed hydrocarbons, and sulphates.
U.S. Pat. Nos. 4,586,367 issued to G. W. Lewis on May 6, 1986 and 4,660,408 also issued to G. W. Lewis on Apr. 28, 1987 teach the importance of adding dilution air through a carefully controlled sampling system and introduce the philosophy of vigorously monitored secondary dilution. Sampling systems of that general class are in use in an effort to satisfy the EPA guidelines on heavy duty truck engine certification compliance. These patents also make reference to the need for eliminating errors in the measurements taken of the diluted exhaust and the diluent air streams and the need for precisely controlling these flow rates.
There has been much consideration of these dilution "tunnel" devices, and including the system of U.S. Pat. No. 4,361,028 issued to S. Kamiya et al. on Nov. 30, 1982 which teaches the relationship between filter pressure differential and accumulated filter mass. Impending legislation will extend the requirements for particulate emissions monitoring to engine sizes well beyond the practical upper limits for "full dilution" systems. The latter term referring to test arrangements wherein the total exhaust gas flow from an engine is mixed with a quantity of diluent air. For example, a locomotive engine of 4,000 Kw output emits approximately 36,000 cubic feet of 450.degree. C. effluent each minute. To test this engine in a configuration such as is described in U.S. Pat. No. 4,586,367 would require a dilution tunnel seven feet in diameter; this is an impractical and economically unacceptable solution.
Investigations into the performance of sampling systems used today indicate excessive variability between governmental bodies, the test organizations, and the engine manufacturers. This variability exerts a negative influence. On the one hand the highlighted discrepancies between the industry test labs translate into competitive advantages for the low-result test labs. And on the other hand, the observed test-to-test variability translates into increased test expenditure because a large number of tests are required to obtain statistically significant results. Although there are several particle mechanisms that influence test-to-test variability, those most significant are particle deposition on the dilution tunnel and tailpipe walls by thermophoresis, by mechanical processes such as diffusion, gravitational sedimentation and turbulence, and by reentrainment of deposited particles and hydrocarbon gas phase exchange of the soluble portion of the diesel exhaust particulate with the deposited wall bound particulate. Therefore, elimination of the deposition mechanism is highly desirable.
The sampling systems of the aforementioned Lewis patents do not treat the phenomena of particle deposition. The long tailpipe used is not insulated nor externally heated and this creates heat transfer conditions conducive to thermophoresis. Additionally, the temperature of the fraction of the sample to obtain secondary dilution is approximately 375.degree. F. The diluted sample is therefore subject to particle loss due to thermophoresis. The collection of material on the walls, rather than on the surface of the filter that is subsequently weighed, can throw off the test results. Furthermore, when a later test is separately run, the agglomerated material is prone to sluff off the walls and be collected by the filter. Moreover, prior systems have often been so large that significant facility support is required.
A dilution tunnel was briefly described in 1974 in an article by Paul M. Giever in "Advances In Instrumentation (Volume 29, Part 3, Paper No. 708) that included a tubular housing, and a tubular probe and a dilution air tube under it within the housing. Air passed radially outwardly from the dilution air tube into the tubular housing and radially inwardly through the probe which was made of stainless steel cloth. A laminar boundary layer of the diluent air shielded the exhaust gas sample from the probe walls while undesirably effecting very little mixing therebetween. A Reynolds number less than 2,000 was called for. Such undesirable flow conditions also restricts the degree of cooling without condensation to a lower limit of 170.degree. F., which is not sufficient for today's standards.
In order to obtain uniformly acceptable results it is mandatory that the withdrawal of the exhaust gas sample and the addition of the diluent air be accomplished at accurately controlled flow rates. Prior gas sampling devices have incorporated relatively unsophisticated flow meters and regulators, and because of this most required continual monitoring by a test operator. In order to assure the accuracy of the test results it was necessary to take a repetitive number of temperature and pressure readings. Thereafter, the collected data had to be mathematically manipulated and correction factors applied because of the crude equipment used. Thus, the existing gas sampling devices have not been sufficiently developed for adoption by commercial industries as a standard.
Clearly, three separate needs are faced by today's diesel engine manufacturers. They are:
1. Reduction in the variability of test results in part through the elimination of thermophoretic deposition of particulates on the walls of the sampling device and corresponding hydrocarbon gaseous phase component exchange with these wall-bound particles; PA0 2. A "down-sized", fully portable system which can yield results equivalent to laboratory systems; and PA0 3. A sampling system that can monitor variable engine operating parameters and can automatically control the rate of exhaust gas withdrawal and can vary the air dilution rate within preselected guidelines within a normal range of operating temperatures and pressures.
The present invention is directed to overcoming one or more of the problems as set forth above.